Detailed Description

The MAX17543 high-efficiency, high-voltage, synchro-

nously-rectified step-down converter with dual integrated

MOSFETs operates over a 4.5V to 42V input. It delivers

compensation across the output voltage range eliminates

the need for external components. The feedback (FB)

regulation accuracy over -40°C to +125°C is ±1.1%.

The device features a peak-current-mode-control

architecture. An internal transconductance error amplifier

produces an integrated error voltage at an internal node,

which sets the duty cycle using a PWM comparator, a high-

side current-sense amplifier, and a slope-compensation

generator. At each rising-edge of the clock, the high-

side MOSFET turns on and remains on until either

the appropriate or maximum duty cycle is reached, or

the peak current limit is detected. During the high-side

MOSFET’s on-time, the inductor current ramps up. During

the second-half of the switching cycle, the high-side

MOSFET turns off and the low-side MOSFET turns on.

The inductor releases the stored energy as its current

ramps down and provides current to the output.

The device features a MODE pin that can be used

to operate the device in PWM, PFM, or DCM control

schemes. The device integrates adjustable-input

undervoltage lockout, adjustable soft-start, open RESET,

and external frequency-synchronization features.

Mode Selection (MODE)

and EN/UVLO voltages exceed the respective UVLO

rising thresholds and all internal voltages are ready to

allow LX switching. If the MODE pin is open at power-up,

the device operates in PFM mode at light loads. If the

MODE pin is grounded at power-up, the device operates

in constant-frequency PWM mode at all loads. Finally,

device operates in constant-frequency DCM mode at light

loads. State changes on the MODE pin are ignored during

normal operation.

PWM Mode Operation

In PWM mode, the inductor current is allowed to go

negative. PWM operation provides constant frequency

operation at all loads, and is useful in applications

sensitive to switching frequency. However, the PWM

mode of operation gives lower efficiency at light loads

when compared to PFM and DCM modes of operation.

PFM Mode Operation

The PFM mode of operation disables negative inductor

current and also skips pulses at light loads for high

efficiency. In PFM mode, the inductor current is forced to

a fixed peak of 750mA every clock cycle until the output

rises to 102.3% of the nominal voltage. Once the output

reaches 102.3% of the nominal voltage, both the high-

side and low-side FETs are turned off and the device

enters hibernation mode until the load discharges the

output to 101.1% of the nominal voltage. Most of the

internal blocks are turned off in hibernation mode to save

quiescent current. After the output falls below 101.1% of

the nominal voltage, the device comes out of hibernation

mode, turns on all internal blocks, and again commences

the process of delivering pulses of energy to the output

until it reaches 102.3% of the nominal output voltage.

The advantage of PFM mode is higher efficiency at light

loads due to lower quiescent current drawn from sup-

ply. The disadvantage is that the output voltage ripple is

higher than in the PWM or DCM modes of operation, and

the switching frequency is not constant at light loads.

DCM Mode Operation

The DCM mode of operation features constant-frequency

operation down to lighter loads than PFM mode by

disabling negative inductor current at light loads instead

of skipping pulses. DCM operation offers efficiency

performance that lies between the PWM and PFM modes.

supply to power the internal blocks and the low-side

should be bypassed with a 2.2µF ceramic capacitor to

SGND. The device employs an undervoltage lockout

falls below 3.8V (typ).

Setting the Switching Frequency (RT)

The switching frequency of the device can be programmed

from 100kHz to 2.2MHz by using a resistor connected

the following equation:

3

RT

SW

21 10

R

1.7

f

×

≅−

open causes the device to operate at the default switching

frequency of 500kHz. See Table 1 for RT resistor values

for a few common switching frequencies. To operate the

MAX17543 at switching frequencies lower than 200kHz, an

MAX17543

4.5V–42V, 2.5A, High-Efficiency,

Synchronous Step-Down DC-DC Converter

with Internal Compensation

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